Recently, adopting Massive-MIMO (Multiple Input Multiple Output) utilizing multi-element antennas in 5G systems (fifth generation mobile communication systems) is under consideration. In order for Massive-MIMO to fully function, a base station desirably performs beam forming, i.e. controlling formation of transmission beams for transmission of a control signal for synchronization in the same manner as transmission of data signals, when performing a cell search to determine an optimum base station for each user terminal to make a connection with. Carrying out such beam forming for transmission of a control signal for synchronization allows reachable distance of the control signal to be extended in the same manner as beam forming for transmission of a data signal, thereby enabling avoidance of ineffective utilization of a communication area for data signals, which ineffective utilization is caused because a user terminal, even after entering into the communication area, cannot properly receive a control signal for synchronization.
Examples of known techniques for adopting beam forming for transmission of such a control signal for synchronization include a method including transmitting a control signal for synchronization from a base station concurrently with forming transmission beams so as to make a rotation of the beam direction; that is, time-divisionally transmitting a control signal for synchronization concurrently with changing in a stepwise manner. (See Patent Document 1)